Concentrated grape must is a product with several applications both for the wine and food industry. In wine production, concentrated grape must is added to the raw grape must whenever the natural concentration of sugars is insufficient to produce high-quality wines, due to the insufficient maturation of the grapes in the vineyard. The insufficient maturation is usually the result of adverse climate conditions, especially when high rainfall occurs during the vintage, but it can be also the result of the anticipation of the vintage. This last practice is commonly performed to avoid the risk of deterioration of the grapes due to the occurrence of rainfall.
Depending on the type of wine produced, grape must concentrate with different specifications of composition can be mixed with raw grape must to increase the final alcohol concentration. Rectified concentrated grape must, which is the outcome of the concentration of the grape must that has been deionised and decolorized, can be used to increase the sugar concentration, and hence, the alcohol potential, of the raw grape must without changing the organoleptic properties of the wine. For this reason, rectified concentrated grape must, which is a mix of the natural grape sugars with a concentration between 65 and 70° Brix, can be used in the production of every type of wine.
For the some European countries where chaptalization is forbidden, the addition of concentrated grape must is the only available method allowed to increase the alcohol potential of raw grape musts. Even for the countries where chaptalization is allowed, there are strong recommendations from the European Commission to replace the saccharose by concentrated grape must. On another side, in officially demarcated regions, the use of concentrated must from grapes of these regions may contribute to maintain the specific organoleptic properties of the wine.
In the food industry, the concentrated grape must is a premium natural sweeter rich in glucose and fructose that can be used instead of saccharose for the formulation of food products that are seen as more natural and healthier by the final consumer.
The conventional process for concentration of grape must is based on vacuum evaporators. In addition, for the production of rectified concentrated grape must, this process is preceded by a complex sequence of ion exchange resins that remove almost completely organic acids, polyphenols and delicate compounds such as aromas precursors. The evaporation process can increase the total solids concentration of grape must until about 70° Brix, but does not preserve the aromatic precursors of the wine and other delicate constituents, which are degraded thermally or by oxidation. Furthermore, due to the change of phase of the evaporated water, the evaporation is an energy intensive process. Additionally, for the production of rectified concentrated grape must, the sequence of ion exchange resins is complex, expensive and generates high volumes of liquid effluents and solid wastes.
Different types of processes have been proposed to produce concentrated grape must or rectified concentrated grape must. The concentrated grape must is usually produced by using vacuum evaporators [1,2]. In the case of the rectified concentrated grape must, the organic acids, ions and aromas are additionally removed in a complex sequence of ion exchange resins [3,4]. The main disadvantages of the evaporation process are: the irreversible modification of the grape must composition due to the increase of the temperature, the loss of volatile fragrances and the high energy consumption due to the change of phase of the removed water. In the case of production of rectified concentrated grape must, there is not only the loss of organoleptic properties of the grape must, but also the environmental problem of discharge of high volumes of effluents produced during the regeneration of the ion exchange resins.
The reverse osmosis (RO) has been proposed to produce concentrated grape must to be used for wine production [5], using RO membranes with a sodium chloride rejection coefficient higher than 97.5% in plate-and-frame modules. More recent studies [6,7] have shown, however, that the productivity of RO can be very low due to the membrane fouling, caused by the precipitation of potassium bitartrate.
The nanofiltration, as a unit operation controlled by steric hindrance mechanisms associated simply to the molecular weight cut-off limit of the membrane, has been investigated in its basic capacity of concentrating grape must, in alternative to the reverse osmosis and with the advantage of operating at lower transmembrane pressures [7-9]. In the present invention, the nanofiltration is primarily controlled by mechanisms of electrostatic interactions that make possible the preferential transport of ionic species and the fractionation of mixtures of neutral solutes (sugars) and organic acids that enable the simultaneous concentration and partial rectification of the grape must in a single operation.